The present invention relates to single-channel-per-carrier (SCPC) multiple-access communication systems and more particularly to those including a voice-activated transmission power controller and usable in satellite communications.
For satellite communications, single-channel-per-carrier multiple-access communication systems (often called "SCPC" (Single-Channel-Per-Carrier) systems in the broad sense of the term) are in wide use as communication systems in which communication channels can be effectively established among a large number of earth stations. In such systems, each communication channel is served by an independent carrier for signal transmission therethrough as in the SPADE and SCPC systems. (In the latter or the SCPC system in its narrow sense, a carrier frequency is permanently assigned to each communication channel.)
In such single-channel-per-carrier multiple-access communication system, a voice activation technique is utilized to allow the transmission equipment for each communication channel to transmit carrier waves only when the channel is in connection and a voice channel signal is actually input thereto, for the purpose of minimizing power consumption of the satellite and preventing occurrence of intermodulation due to saturation of the satellite transponder. Accordingly, in the conventional system, a gate circuit has been arranged on the output side of the modulator for ON/OFF control of the modulated carrier wave and is controlled by a combination of the output of a voice signal detector, which detects the presence or absence of a voice channel signal, and the channel connection signal for connection control of the channel. Any leakage signal from such gate circuit should be minimized since it tends to adversely affect other channels as a factor causing unwanted wave radiation during nonconnection periods of the channel, (i.e., when the channel is not being utilized with no demand for communication therethrough). This not only makes the structure of the gate circuit complicated but also involves the disadvantage that, in cases where phase-shift keying digital modulation is employed as in the so called "SPADE" system, different sync systems (including carrier recovery and bit timing circuits) in the receive demodulator are complicated in design and structure since voice activation generates burst signals when the electromagnetic wave radiation disappears almost completely in the absence of voice input.
For the details of such SCPC communication system, reference may be had to:
(1) T. Takahashi et al: "TRANSMISSION SUBSYSTEM FOR SPADE TERMINAL", 2nd International Conference on Digital Satellite Communication, Paris (FRANCE), Nov. 28-30, 1972 pp. 127-143.
(2) John G. Puente et al: "Multiple-Access Techniques for Commercial Satellites", PROCEEDINGS OF THE IEEE, Vol. 59, No. 2, pp. 218-229, Feb. 1, 1971.
(3) C. J. Wolejsza, Jr. et al: "PSK MODEMS FOR SATELLITE COMMUNICATIONS", INTELSAT/IEE International Conference on Digital Satellite Communication, pp. 127-143, Nov. 25-27, 1969.